1. Field of the Invention
The present invention relates to a high-pressure discharge lamp. More specifically, the present invention relates to a high-pressure discharge lamp which, even after being lit for a considerably long time, does not have problems such as deformation of the bulb thereof made of quartz glass and decrease in brightness of the discharge lamp due to devitrification and blackening of the quartz glass bulb.
2. Description of Related Art
In general, high-pressure discharge bulbs have a structure in which each electrode of a pair of electrodes (i.e., an anode and a cathode) is disposed so as to be opposite each other in a quartz glass bulb, which includes an expanded portion for luminescence and a sealing portion, and the anode and the cathode are jointed by, for instance, welding with molybdenum foil. Also, the sealing portion of the quartz glass bulb is airtight-sealed by, for example, welding with molybdenum foil. A gas for assisting an electric discharge, such as mercury vapor, is contained in the expanded portion for luminescence of the quartz glass bulb which has been sealed airtightly.
When such a high-pressure discharge lamp is used, a large amount of ultraviolet rays having short wavelengths are emitted because of the considerably high vapor pressure of the gas contained in the lamp and of the high pressure applied to tube walls. As a result, the temperature of the quartz glass bulb is increased significantly due to the absorption of the ultraviolet rays having short wavelengths. Thus, if natural quartz glass is used for the quartz glass bulb in a conventional manner, there are problems associated with the use thereof, such as deformation, devitrification, and blackening of the bulb, which shorten the lifetime of the bulb, because a minute amount of impurities is always contained in even high-quality natural quartz glass.
To solve the above-mentioned problems, for instance, the Japanese Unexamined Patent Application, First publication No. 6-187944 discloses a light emitting tube for a high pressure discharge lamp including a pair of electrodes and a composite quartz glass tube for sealing the pair of electrodes. An outer layer of the composite quartz glass tube is made of natural quartz glass having a viscosity of 1014.8 poise or more at 1025xc2x0 C. and an inner layer thereof is made of synthetic quartz glass which includes impurities including, at least, alkali metals. The amount of each of alkali metal species is 0.2 ppm or less.
However, it is difficult to carry out a sealing process for the above-mentioned light emitting tube since the composite quartz glass tube has a dual structure and the dispersion of the inner diameter size thereof is quite large. As a result, the dispersion in the inner volume among the quartz glass bulb increases and the pressure of the discharge lamp is significantly varied. Also, since the alkali components of the natural quartz glass are dispersed towards the synthetic quartz glass during the process constructing the dual structure thereof, little suppression effect for the devitrification and the blackening is obtained. Moreover, the cost for producing the above-mentioned light emitting tube is expensive and, hence, not economical.
On the other hand, a quartz glass having a viscosity of 1013.0 poise or more at 1200xc2x0 C. whose hydroxyl group concentration is 10 ppm or less is known as a highly thermostable quartz glass used for the semiconductor industry, such as core tube made of a quartz glass (refer to the Japanese Unexamined Patent Application, First publication No. 8-81226.) However, it had not been known that if a quartz glass bulb which is formed as a one-piece unit by using a synthetic quartz whose viscosity is about 1013 poise or more at about 1200xc2x0 C. is used as a high-pressure discharge lamp, the deformation of the quartz glass bulb or the decrease in brightness of the discharge lamp due to the devitrification or the blackening of the bulb may be prevented even if the discharge lamp is used for a long time at about 1000-1200xc2x0 C. under a high pressure of about 100-180 atmospheres.
The inventors of the present invention, after pursuing diligent studies to solve the above-mentioned problems, have discovered that it is possible to prevent the deformation of the quartz glass bulb and the decrease in brightness of the discharge lamp due to the devitrification and the blackening of the quartz glass bulb, even after being lit for a considerably long time, if a synthetic quartz glass having a viscosity of about 1013 poise or more at about 1200xc2x0 C. is used to form a quartz glass bulb as a one piece unit for the high-pressure discharge lamp. Also, since the quartz glass bulb is formed in a single piece, the high-pressure discharge lamp may be easily produced.
It is therefore an object of the present invention to provide a high-pressure discharge lamp which, even after being lit for a considerably long time, does not have problems such as the deformation of bulb thereof made of quartz glass and the decrease in brightness of the discharge lamp due to the devitrification and the blackening of the quartz glass bulb. Another object of the present invention is to provide such a high-pressure discharge lamp which is readily produced.
Accordingly, the present invention provides a high-pressure discharge lamp including: a quartz glass bulb; conductive elements, the conductive elements being airtightly sealed at sealing portions of the quartz glass bulb; and a pair of electrodes, each of the pair of electrodes being disposed so as to be opposite each other and each electrode being connected to one of the conductive elements; wherein the quartz glass bulb is formed as a one piece unit by using a synthetic quartz whose viscosity is about 1013 poise or more at about 1200xc2x0 C.
The present invention also provides a high-pressure discharge lamp wherein the conductive element is molybdenum.
The present invention also provides a high-pressure discharge lamp wherein the amount of alkali metals contained in the quartz glass bulb is about 0.01 ppm or less.
The present invention also provides a high-pressure discharge lamp wherein the amount of hydroxyl group contained in the quartz glass bulb is about 1 ppm or less.
According to the present invention, it becomes possible to prevent the deformation of the quartz glass bulb and the decrease in brightness of the discharge lamp due to the devitrification and the blackening of the quartz glass bulb, even after being lit for a considerably long time. Also, the high-pressure discharge lamp may be easily produced because the quartz glass bulb is formed in a single piece,.